Re: [PATCH] mm/gup: Do not return 0 from pin_user_pages_fast() for bad args

[David Hildenbrand <david@redhat.com>]

On 19.06.23 20:27, Jason Gunthorpe wrote:
> These routines are not intended to return zero, the callers cannot do
> anything sane with a 0 return. They should return an error which means
> future calls to GUP will not succeed, or they should return some non-zero
> number of pinned pages which means GUP should be called again.
> 
> If start + nr_pages overflows it should return -EOVERFLOW to signal the
> arguments are invalid.
> 
> Syzkaller keeps tripping on this when fuzzing GUP arguments.
> 
> Reported-by: syzbot+353c7be4964c6253f24a@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/000000000000094fdd05faa4d3a4@google.com
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>   mm/gup.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index bbe4162365933e..36c587fec574fd 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2969,7 +2969,7 @@ static int internal_get_user_pages_fast(unsigned long start,
>   	start = untagged_addr(start) & PAGE_MASK;
>   	len = nr_pages << PAGE_SHIFT;
>   	if (check_add_overflow(start, len, &end))
> -		return 0;
> +		return -EOVERFLOW;

I'm curious if there is any sane use case where that could actually 
trigger. Smells like something that should be a WARN_ON_ONCE(), but 
maybe some callers simply pass through what user-space gave them.

Anyhow.

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb